I assume that you mean that the engine won't crank when you hit the START position on the ignition switch. If so, and you're sure about the battery, I suspect that the starter is failing. Check that you have battery voltage at the big starter cable, and battery voltage at the smaller START wire at the starter when your assistant turns the key to START. If so, and the starter doesn't run, then it's faulty.
If you want to find a Nissan specialist, here's our directory link for you:
https://repairpal.com/directory?address=17109&car_brand_names=Nissan

...2002. Now it has over 150k miles on it.
2. Once or twice a year since she bought it, the car won't start (engine will not even crank) unless she shifts the automatic transmission into neutral and back...